This invention relates to a method of correcting the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine so as to take atmospheric pressure into account. More particularly, the invention relates to a method of correcting the air-fuel ratio for atmospheric pressure so as to avoid a leaner air-fuel ratio from being brought about when the engine is operating under a small load.
A known method of controlling the supply of fuel to an internal combustion engine having a fuel injection device entails setting a period of time during which the valve of the injection device is to be opened to a basic value determined in dependence upon engine rotational speed and absolute pressure in the engine intake pipe, and correcting the set basic period of time during which the valve is to be opened in dependence upon sensed values of operating parameters (e.g. engine temperature, throttle valve opening, atmospheric pressure) representing operating conditions of the engine, thereby deciding an amount of fuel supply in such a manner that the air-fuel ratio of the mixture supplied to the engine will attain a desired air-fuel ratio, e.g. a stoichiometric mixture ratio (e.g. Japanese Provisional Patent Publication (Kokai) No. 58-85337).
When an internal combustion engine is operated under a low atmospheric pressure such as exists at a high altitude, the drop in atmospheric pressure is accompanied by a decline in engine back pressure, namely the pressure in the exhaust pipe. A consequence of the reduction in back pressure is a higher engine exhaust efficiency, as a result of which the charging efficiency rises. This in turn causes a leaner mixture to be supplied to the engine, unless a countermeasure is taken to compensate for the drop in atmospheric pressure. This tendency toward a leaner mixture becomes more pronounced the lower the rotational speed of the engine and the smaller the engine load. More specifically, since the back pressure of the engine is very low when the engine is operating under a small load as during rotation at low speed, the back pressure is readily influenced by a change in atmospheric pressure. The smaller the engine load becomes in such case, the greater the rate at which back pressure declines with respect to a drop in atmospheric pressure. This results in a correspondingly higher exhaust efficiency and, hence, a correspondingly greater rate of increase in charging efficiency. The end result is a mixture which will become more lean so long as the amount of fuel supplied is constant.
The conventional fuel supply control method mentioned above attempts to deal with this problem by calculating a correction value, namely a value which corrects the basic value of valve opening period for atmospheric pressure, based upon the prevailing value of atmospheric pressure and the intake pipe absolute pressure value, which is indicative of the magnitude of engine load, thereby deciding a correction value that conforms to operating conditions of the engine. However, the conventional method relies upon a complicated arithmetic expression in order to calculate the correction value, as a result of which the calculation processing requires a considerable period of time. The unfortunate consequence is a control delay that renders the method impractical for use.